15,000 litres, is only 15 cubic metres. About the size of two small toilets. I wouldn't have thought you'd need much more than a couple of square feet to move enough air.
Their solution to the negative pressure issue is simple and effective. Whilst I'd expect the NSW 737 to have a more elegant solution, I expect that it's still pretty simple.
Airliners do have negative pressure vents. The 747's was in forward fuselage on the port side, roughly at door 2.
